In surveillance systems of the type referred to above, the markers have generally been single status, i.e., they have not been able to be reused, i.e., repeatedly, deactivated and subsequently reactivated. Such a limitation has, for the most part, restricted the use of such systems to applications in which the marker is physically removed from the object when detection thereof is no longer desired, such as at the point of sale. It has also been proposed to physically and irreparably destroy or alter the resonant circuit within the marker, such as by a fusible link which is melted so as to open a portion of the circuit. (See Lichtblau, U.S. Pat. No. 3,810,147). Such a scheme has not found commercial acceptability, possibly due to the expense of the markers, which become useless after the circuit is thus irreparably destroyed or altered.
As the resonant circuits used in the markers are readily affected by a conductive sheet placed in close proximity, it is also known to selectively deactivate such markers by positioning such a sheet i.e., an aluminum foil, next to the marker. For example, such a marker could be concealed within the UPC label or beneath the pocket in a library book into which a "check-out" card is to be inserted. A conductive foil concealed within the check-out card or within a separate paste-on label would then be provided. Obviously, that scheme requires separate deactivatable components, and may be impractical for use in many situations.
One suggestion for providing a deactivatable/reactivatable RF marker is set forth in U.S. Pat. No. 3,493,955 (A. J. Minasy). In that patent, it is proposed that a small, non-conductive but magnetizable element such as a ferrite can be positioned on or close to a coil in the marker. It is stated in that patent that normally the ferrite will be in its non-magnetic state and will have no effect on the operation of the marker, but that when it is desired to deactivate the marker, the ferrite be brought to its magnetic state, thereby interfering with the electromagnetic fields of radio waves in the vicinity, and hence effectively preventing operation of the device. A large electromagnet is proposed to be used to switch the ferrite back and forth between its magnetic and non-magnetic state. Such a concept is not known to have ever been successfully utilized.
U.S. Pat. No. 4,063,229 (Welsh et al) depicts yet another type of EAS system wherein a marker or tag containing an electrically non-linear element such as a diode is used to generate harmonics of a transmitted microwave signal, typically at 100 or 915 MHz. Harmonics, typically second order at 200 or 1830 MHz, are then detected. Such a marker does not have an intrinsic resonant frequency, and the diode is provided with an antenna tuned to the transmitted frequency to enhance the absorption of energy and the transmission of harmonic radiation. That patent (Col. 18, line 47ff, and FIGS. 10 and 11) also suggests that the tags can be made deactivatable by providing layers of two ferrites adjacent inner and outer antenna loops joined together via a non-linear capacitor such as a reverse biased diode. The first ferrite layer (407) is proposed to be a high retentivity, permanently magnetizable ferrite, while the second layer (408) is a soft low retentivity ferrite. The tag is said to be activatable by magnetically saturating the first layer. Flux from that layer returns through the second layer which is thereby also saturated, and the inductances of the antenna loops are, therefore, generally unaffected. To deactivate the tag, the first ferrite layer is demagnetized. The second layer then possesses high permeability and increases the inductance of the loops to about twice their former value, thus reducing the reaction fields below a detectable level.
FIGS. 15 and 16 and the accompanying description (Column 20, lines 12-24) of the Welsh '229 patent suggest a deactivatable tag in which a single tuned loop circuit is used which resonate at the fundamental system frequency, and in which no non-linear element is provided. First and second ferrite layers (407 and 408) are provided as in the embodiments shown in FIGS. 10 and 11. It is there suggested that such an embodiment could be employed in applications in which selectivity does not pose a problem because no articles are present which are sufficiently conductive to distort the applied fundamental frequency field through creation of eddy currents. Notwithstanding the reasonable commercial success enjoyed by such diode containing tags and associated systems transmitting at microwave frequencies, it is not believed that deactivatable tags as described in the '229 patent have ever been found to be practical.
In another, totally different, type of electronic article surveillance system, magnetically deactivatable and reactivatable markers have been successfully employed for a number of years. See, for example, U.S. Pat. No. 3,665,449, Elder & Wright. Such systems utilize a marker which is itself magnetic, comprising an elongated strip of low coercive force, high permeability ferromagnetic material, adjacent to which is positioned at least one piece of a higher coercive force, permanently magnetizable material. When the magnetization in such a strip is reversed by a low frequency alternating magnetic field produced in an interrogation zone, detectable harmonics of that frequency are generated. In direct opposite to that suggested by Welsh et al ('229), such a magnetic marker is deactivated by magnetizing the higher coercive force, permanently magnetizable material. The magnetized material magnetically biases the low coercive force material and prevents the magnetization therein from reversing due to the alternating field present in the interrogation zone, thus preventing its detection, i.e., deactivating it. The deactivatability thus provided has greatly contributed to the significant commercial success enjoyed by such systems over the past decade. The absence of a practical reversible-deactivation capability has, on the other hand, appreciably restricted the areas in which the other systems could be used and has thereby lessened the commercial success of such systems.